That really must have been a critical failure on the part of Obi-Wan at that point in the movie. How else could he - a highly trained Jedi skilled
in hand to hand combat - possibly have forgotten that he was still attached to Jango by a cable and push him over the edge of a precipice?

Transcript

Obi-Wan: I rush Jango and try to tackle him.Obi-Wan: Uh oh. 1.GM: You push him off the edge of the landing platform.Obi-Wan: Well that's not so bad.[SFX]: K-paff!GM: He's still attached to you by the cable.Obi-Wan: Ah.[SFX]: Yank! {Obi-Wan is pulled off the landing platform after Jango}[SFX]: Skrrreeeee! {Jango slows his descent using metal claws}
{Jango has stopped on the slope, Obi-Wan is dangling from the cable}Jango Fett: Hmmm, still 90 seconds to go. I better let you go and get on with my plans.
{Jango cuts Obi-Wan free}
{Obi-Wan falls, landing on a walkway at a lower level}GM: By the time you get back to the platform, he's taking off.[SFX]: WOOMSSHH! {the Fetts take off}Obi-Wan: I throw my listening device on to his ship.
{Obi-Wan throws tracking device on to ship}Obi-Wan: What are they saying?Jango Fett: Oh look, Kenobi's attached a bug.